Abstract: A hydraulic vibration tester comprises an actuator comprising a piston reciprocable in a cylinder connected with a vibration testing table and a direct drive type servo valve for supplying pressure fluid alternately to opposite ends of the cylinder. The servo valve comprises a spool valve member reciprocable in the bore of a valve body by a magnetic coil supplied with alternating current and located in a direct current magnetic field. At opposite ends of the spool valve member there are coned disc springs acting to bias the spool valve member to a median position and acting to allow only an axial reciprocating movement of the spool valve member at a frequency in the range of 650 Hz to 2000 Hz. The passages connecting the servo valve with the activator have a resonant frequency in the same range so as to achieve an effective vibration test in the high frequency range.